1. Field of the Invention
The invention relates to aircraft recovery systems. More particularly, this invention relates to a system and method for recovery of a smaller unmanned aerial vehicle (micro aerial vehicle (MAV)) by a larger unmanned aerial vehicle.
2. Background Art
Unmanned aerial vehicle (UAV) technology has proven to be an essential surveillance tool. UAV technology is so pervasive, that unmanned aircraft are not uncommon on the daily television news. Though larger UAVs are typically shown, the variety of vehicles in the field is actually much larger. In general terms, though, the types of aircraft can be divided into two different groups. In the first group are the long range, high altitude, and relatively high speed UAVs, with optical sensor and/or synthetic aperture radar (SAR) payloads. In the second group are micro air vehicles (MAVs). MAVs can be hand launched, have limited endurance, operate at low altitude, and travel at lower speeds. FIG. 1 illustrates a large unmanned aerial vehicle and a micro aerial vehicle flying side-by-side, thereby demonstrating their relative size and speed. Nonetheless, MAVs provide high quality, narrow field-of-view imagery that can be useful in law enforcement and border patrol.
A system or method that synergistically combines the advantages of both MAVs and larger UAVs will yield a truly revolutionary capability. For example, a larger UAV can be used as a mother-ship to both deliver and recover MAVs. This creates the capability of rapidly deploying MAVs at much farther distances than ever before. Once deployed, the MAVs can be used to gain high quality narrow field-of-view surveillance, or perhaps land in remote locations and engage in long term monitoring activities. To avoid loss of the aircraft technology and data, to minimize use of personnel in perhaps difficult environment, and to allow reuse of the MAV and sensor payload, the MAV can rejoin the UAV mother-ship. Unfortunately, no satisfactory method truly exists for recovery of MAVs by a UAV at the present time.
If the UAV and MAV had similar flight envelopes, then the problem would be relatively straightforward. This is generally not the case, however, as larger UAV's suitable to the task can fly only as slow as 70-260 knots, while small MAVs have a top speed of only about 30 knots. This disparity in speed means that the capture problem is significant.
One of the earliest studies into a “long line” loiter technique was performed and patented (U.S. Pat. No. 1,829,474) by C. Chilowsky. The invention introduces the idea of an aircraft orbiting at a specified radius and velocity, from which hangs a payload orbiting with a much smaller radius and velocity, and provides some basic calculations for determining the relation between the two radii.
Today the system introduced by Chilowsky is primarily used to collect and deploy equipment and supplies, specifically in remote areas. Missionary groups frequently perform “bucket drops” from a circling plane to provide isolated villages with gifts and provisions. The most widely publicized of these drops is known as Operation Auca. During these drops, a Piper PA-14, piloted by missionary Nate Saint, would circle in a tight spiral over Waodani jungle territory in Ecuador. While in this maneuver, a second man lowered a basket of supplies as gifts to the tribe, who in turn, would send back gifts of their own. This proved to be an effective means of delivery between the two parties and correspondence continued for over three months. The bucket method continues to be used, with many investigations into making it a more efficient and sophisticated air delivery system. For example, such a system could be employed to lower supplies onto a sea-going vessel, among other types.
Another example of a use of the “long line” technique involves the use of two orbiting aircraft as discussed in U.S. Pat. No. 4,416,436. In this system, two aircraft enter into a circular orbit centered around the payload to be transported. A cable with a small weight or drogue shoot is attached to each of the aircraft and lowered to the payload. While the aircraft continue to orbit, the cable is lowered and attached to the payload by ground personnel. The payload is then lifted a safe distance from the surface before a de-orbit procedure is initiated, in which one of the aircraft (aircraft A) reduces its bank angle thereby increasing its orbital radius. With the second aircraft (aircraft B) maintaining its current bank angle, it quickly approaches the first until the two are flying parallel to each other with the payload dangling between them; the cables are then reeled in to reduce drag. At no point during this procedure do the aircraft change their speeds. Upon arriving at the delivery site, the aircraft release the cables and again enter into a circular orbit (by applying different bank angles). Once in the circular pattern, the payload is decelerated with the use of a small parachute and the two aircraft slowly descend until the payload makes contact with the surface.
In addition to the transfer of equipment and supplies, it has also been suggested that this system be employed in the surveillance of enemy territory, as discussed in U.S. Pat. No. 6,705,573. According to this configuration, an aircraft lowers sensors and other equipment from its fuselage until they are a certain distance apart, enabling the observation of the area below the aircraft while the aircraft maintains a safe position.
As is self evident of many of the examples discussed above, each relies on the skill of the pilot to accurately identify the target area, as well as to maintain the location of the payload once circular flight has been established. This task is further complicated by the presence of wind which can result in vertical oscillation of the payload. It is clearly desirable to develop a method to counteract this effect as well as one that will place less emphasis on the skill of the pilot or controller. One possible solution to this problem was proposed in 1998 by using electronic sensors and detection devices as discussed in U.S. Pat. No. 5,722,618. Near-stationary positioning of the payload can be accomplished in a number of ways. In the first, a GPS sensor positioned on the aircraft can be used to alter the aircraft's trajectory to ensure the center of the circular flight path coincides with the target area; alternatively the sensor may be placed on the payload and the aircraft can be repositioned to match the payload coordinates with those of the desired location. A third approach again places a sensor on the payload capable of detecting signals from the target and determining the relative position between them. Near-stationary vertical motion is established by situating an altitude sensor on the payload, the output of which is delivered to the orbiting aircraft. By monitoring the payload altitude, the aircraft can either change its velocity, altitude, or the length of the line to maintain desired height. In addition, the positioning of thrust devices on the payload could enable it to move independently of the aircraft to which it is attached (for small increments).
Research into long-line placement of objects continues undiminished. For example, Pavel Trivailo and a team of engineers at the Royal Melbourne Institute of Technology in Australia are currently exploring new applications for the “bucket drop” maneuver, working on an automated system that will allow them to pick up and place payloads with minimal impact. Such a system will be highly beneficial for rescue missions and deliveries in areas in which human access is extremely difficult. To this end the team performed numerous simulations based on fuzzy logic, the Chebyshev-pseudospectral method, and optimal flight configurations to analyze the dynamic properties of a towed payload, specifically the control laws that govern the cable and the cable-payload assembly. In performing these simulations, they were able to identify the factors that contribute most to the stabilization of the cable (length, thickness, aircraft speed, etc.). With these factors determined, they developed a system controller that monitors and adjusts the position of the payload by automatically letting out or reeling in the tow line.
It should be noted that while all of the above prior art examples appear to be, at the least, interesting uses of long-line placements of objects, none of the prior art recognizes, teaches, suggests, or comprehends that extending a long line from an aircraft can be used for anything more than placement and/or retrieval of objects on or near stationary locations. Therefore, notwithstanding all of the above, a need still exists for the safe capture of micro UAVs (MAVs) by larger UAVs such that the needs of various police, firefighting, border patrol, drug enforcement, and military agencies are met.